Cannot save(serialize) a double array of a custom class

I can save and access an array of the custom data type, I can also access the double array of built in types such as int, but the double array of my custom type gives an " object reference not set to an instance of an object" error message in the line "bF.Serialize(file, data);". although all three object references are instantiated.

I can easily work around this, but that would result in less clean code, I could for example make a second custom class which houses the double array, which resolves the problem. I, however would like to know why my code isn't working as expected.

For clarification: Double array of int:

 public int[] arr1;
     public int[] arr2;
     void Save() {
         BinaryFormatter bF = new BinaryFormatter();
         FileStream file = File.Open(Application.persistentDataPath + "/data.dat", FileMode.Create);
         int[][] data = new int[2][];
         data[0] = new int[] { 1, 2, 3 };
         data[1] = new int[] { 4, 5 };
         bF.Serialize(file, data);
         file.Close();
     }
     void Load() {
         BinaryFormatter bF = new BinaryFormatter();
         FileStream file = File.Open(Application.persistentDataPath + "/data.dat", FileMode.Open);
         int[][] data = (int[][])bF.Deserialize(file);
         file.Close();
         arr1 = data[0];
         arr2 = data[1];
     }

Works like a charm.

• array of custom class:

     void Save() {
               BinaryFormatter bF = new BinaryFormatter();
               FileStream file = File.Open(Application.persistentDataPath + "/data.dat", FileMode.Create);
               CustomClass[] data = new CustomClass[]{new CustomClass(1), new CustomClass(2), new CustomClass(3)};
               bF.Serialize(file, data);
               file.Close();
           }
           void Load() {
               BinaryFormatter bF = new BinaryFormatter();
               FileStream file = File.Open(Application.persistentDataPath + "/data.dat", FileMode.Open);
               CustomClass[] data = (CustomClass[])bF.Deserialize(file);
               file.Close();
               arr1 = data;
           }
  
  

No problems here

• Double array of custom class:

     public CustomClass[] arr1;
       public CustomClass[] arr2;
       [System.Serializable]
           public class CustomClass {
               public CustomClass(int i) {
                   integer = i;
               }
               public int integer;
           }
       void Save() {
               BinaryFormatter bF = new BinaryFormatter();
               FileStream file = File.Open(Application.persistentDataPath + "/data.dat", FileMode.Create);
               CustomClass[][] data = new CustomClass[2][];
               data [0] = new CustomClass[]{new CustomClass(1), new CustomClass(2), new CustomClass(3)};
               data [1] = new CustomClass[]{new CustomClass(4), new CustomClass(5)};
               bF.Serialize(file, data);
               file.Close();
           }
           void Load() {
               BinaryFormatter bF = new BinaryFormatter();
               FileStream file = File.Open(Application.persistentDataPath + "/data.dat", FileMode.Open);
               CustomClass[][] data = (CustomClass[][])bF.Deserialize(file);
               file.Close();
               arr1 = data[0];
               arr2 = data[1];
           }
  
  

Won't work...